The present invention relates to aqueous polymeric dispersions, a method of making aqueous polymeric dispersions, and coating compositions containing aqueous dispersions.
Color-plus-clear coating systems involving the application of colored or pigmented base coat to a substrate followed by the application of transparent or clear topcoat to the base coat are becoming increasingly popular as original finishes for automobiles. Automotive base coats containing metallic pigments such as aluminum flake are generally utilized to obtain a desired lustrous appearance.
Over the past several years, there has been a trend in the automotive industry toward the reduction of atmospheric pollution caused by the volatile organic solvents which may sometimes be emitted during the painting process. One approach to emissions control has been the use of waterborne coatings compositions as the pigmented color coat or base coat in the color-plus-clear system.
A major problem associated with waterborne base coats containing metallic pigment is that without the proper rheology properties, an undesirable appearance of non-uniform areas of light and dark color can result after application of the base coat to the substrate because the metallic flakes may lack the proper orientation or not be uniformly orientated across the surface of the substrate. This condition of non-uniformity of color is commonly referred to as mottle or mottling. It is known that cellulose esters can be used as rheology control agents in both solventborne and waterborne coatings to control metallic flake orientation.
U.S. Pat. No. 5,334,638 describes an aqueous dispersion containing a hydrolyzed cellulose ester and an acrylic resin, having free acid functionalities that are at least partially neutralized, which can be used in a coating composition. The cellulose ester was blended with the acrylic resin to form a dispersion in water. The acrylic resin is not polymerized in the presence of the cellulose ester.
U.S. Pat. No. 5,384,163 describes the use of cellulose esters modified with anhydrides of dicarboxylic acids as rheology control agents in waterborne base coats.
U.S. Pat. No. 4,970,247 describes aqueous dispersion polymers containing nitrocelluloses and other polymerizable monomers. The dispersions are useful in nitrocellulose lacquers that will dry without cloudiness. Nitrocellulose lacquers are typically too high in volatile organic content to be used as automotive coating compositions because of environmental regulations.
U.S. Pat. Nos. 3,953,386 and 4,011,388 describe aqueous homogeneous polymer emulsions containing cellulosic esters and acrylic polymers. The acrylic monomers are chosen so as to obtain polymers that are substantially non-grafted and non-crosslinked. When dispersions containing non-crosslinked polymers in the dispersed particles are used in base coat compositions of color-plus-clear coating systems, there can be problems associated with "soak in" of the subsequently applied clear coat. "Soak-in" occurs when the solvent from the subsequently applied clear coat partially dissolves the non-crosslinked polymers, thereby allowing metallic flakes in the base coat to move and disorientate themselves. Another consequence of "soak-in" can be low gloss or sometimes a hazy appearance of the resultant film.
In the automotive industry, after a car body has been coated with an organic coating composition, the body is then "trimmed out," i.e., the assembly of the car is completed by attaching the interior parts and the exterior parts, including the engine, drive train, wheels and tires, windshields, backlights, and the glass side windows. During this assembly process, the coating on the surface of the body can be damaged thereby necessitating a repair or repainting of the affected area of the body. Because the car has been essentially totally assembled, the same coating process used to originally coat the body cannot be used to repair the coated body. Generally, a fully assembled vehicle cannot be subjected to the relatively high cure temperatures used to cure the original coatings because the vehicle contains many plastic parts that would melt or become distorted or inoperable if subjected to those elevated temperatures. As a result the industry has developed a "low bake repair" process where the assembled vehicle is repaired with coatings that are capable of or that have been modified to cure at a low temperature such that the plastic parts on the vehicle will not melt or distort. The original base coats normally are used as the repair coatings to minimize any variation in color between the repaired areas and the rest of the body. The original coatings are sometimes modified to cure at "low bake" temperatures by adding additional catalyst to the coatings. It has been found that at the low cure temperatures used in the "low bake repair" process it is very difficult to obtain equivalent properties as an original finish coating, such as humidity resistance, appearance, adhesion, and chip resistance.
It is desirable, therefore, to have aqueous coating compositions that have excellent resistance to mottling and also exhibit acceptable humidity resistance, appearance, adhesion, and chip resistance when used in a "low bake repair" process.